The invention relates to optical fluoride crystals, and particularly to optical fluoride crystals, such as calcium fluoride, which have high transmission levels to below 300 nm light, such as produced by excimer lasers. In particular the invention relates to making optical fluoride crystalline laser components with improved flatness final polished surfaces. The invention relates to a calcium fluoride final polishing agent of colloidal silica soot. The invention also relates to a unique method of polishing fluoride crystal optical surfaces using fused silica soot.
Applications of colloidal suspensions for polishing materials has become an exceedingly critical aspect of the final part formation of optical components and blanks/preforms thereof. Silica and alumina colloids are formed through various techniques and typically require expensive precursor materials in order to ensure the highest purity products. Solutions are stabilized with buffer systems to pH and solids loading values that result in optimal surface finish attainment. Particle size distribution can be adjusted to control the final surface finish, as well as the ability to clean residue abrasive particles from workpiece surfaces after processing.
The level of final polish currently available is not good enough for optical fluoride crystalline laser components. Especially critical is that final polishing with present colloidal abrasive particles greatly degrades the flatness of the optical surface of the optical fluoride crystalline laser component being made and polished. Small surface imperfections easily distort the laser light to be manipulated by the crystal component and small particle size abrasives have a low long removal rate. Also, hard, optical quality pads scratch the surface when used with conventional non-spherical small particle size abrasives.
The invention includes a method of making optical fluoride crystal components, preferably optical fluoride crystal laser components and optical lithography systems such as utilized in below 200 nm excimer lasers and optical lithography systems. The method includes providing an optical fluoride crystal having an initial polish finished surface with an initial finish surface roughness greater than 1 nm RMS and an initial finish flatness, providing an optical fluoride crystal final polishing solution, said final polishing solution comprised of a plurality of particulate abrasive agent colloidal silica soot, and polishing with said provided optical fluoride crystal final polishing solution said provided optical fluoride crystal initial polish finished surface to a final polished surface having a reduction in flatness no greater than 50% of said initial finish flatness, and said final polished surface having a final polished surface roughness less than 1 nm RMS. The invention provides a beneficial final polish of 0.5 nm RMS or better to a fluoride crystal that has an initial polished surface less than 5 nm RMS. The polishing solution can be aqueous, preferably in the pH range of 2-12, and more preferably 9-12. In an alternative embodiment the polishing solution is non-aqueous, such as based on ethylene glycol or kerosene. The spherical soot particles provide beneficial polishing of fluoride crystal with little to no scratching and a finer surface finish. The soot particle polish provides better retention of form and figure, and particularly flatness for flat surface optical elements such as laser components, with the belief that the soot particles form a beneficial polishing barrier (due to particle size and size distribution) between the fluoride crystal surface and the polishing pad utilized in the polishing of the fluoride crystal surface. The soot particle polish is preferably applied with a synthetic polishing pad, with the soot providing improved removal rates and preventing damage from polishing pad contact with the fluoride crystal surface, preferably with the soot particles filling the pad pores to form a surface of spherical soot particles that the crystal surface is abraded against.
The invention includes a method of polishing an optical fluoride single crystal, preferably a calcium fluoride single crystal. The method includes providing an optical fluoride crystal having an optical transmission surface, providing a final polishing fused silica soot solution, said final polishing solution comprised of a plurality of particulate abrasive agent colloidal solid sphere fused silica soot particles, and polishing said optical fluoride crystal optical transmission surface with said final polishing colloidal solid sphere fused silica soot solution to provide a polished optical fluoride crystal optical transmission surface.
The invention includes a method of making an optical fluoride crystal polishing slurry. The method includes providing a plurality of particulate abrasive agent solid sphere fused silica soot particles, providing an optical fluoride crystal polishing pre-slurry solvent, dispersing said particulate abrasive agent colloidal solid sphere fused silica soot particles in said optical fluoride crystal polishing pre-slurry solvent to form an optical fluoride crystal polishing slurry. The invention includes an optical fluoride crystal polishing slurry and an optical fluoride crystal final polishing particulate agent comprising colloidal silica soot having an average particle size ranging from above 50 nm to 500 nm, preferably an average particle size ranging from 100 nm to 400 nm, and more preferably an average particle size ranging from 250 nm to 350 nm. Preferably the colloidal silica soot has a spherical morphology and a specific surface area of 20 m2/g or less.